1. Field of the Invention
The present invention relates to an organic electroluminescent device and particularly to a compensated pixel driver circuit thereof.
2. Description of Related Art
An organic electro-luminescent device (OELD) consists of a light emitting polymer (LEP) layer sandwiched between an anode layer and a cathode layer. Electrically, this device operates like a diode. Optically, it emits light when forward biased and the intensity of the emission increases with the forward bias current. It is possible to construct a display panel wide a matrix of OELDs fabricated on a transparent substrate and with one of the electrode layers being transparent. One can also integrate the driving circuit on the same panel by using low temperature polysilicon thin film transistor (TFT) technology.
In a basic analog driving scheme for an active mat OELD display, a minimum of two transistors are required per pixel (FIG. 1): T1 is for addressing the pixel and T2 is for converting the data voltage signal into current which drives the OELD at a designated brightness. The data signal is stored by the storage capacitor Cstorage when the pixel is not addressed. Although p-channel TFTs are shown in the figures, the same principle can also be applied for a circuit with n-channel TFTs.
There are problems associated with TFT analog circuits and OELDs do not act like perfect diodes. The LEP material does, however, have relatively uniform characteristics. Due to the nature of the TFT fabrication technique, spatial variation of the TFT characteristics exists over the entire panel. One of the most important considerations in a TFT analog circuit is the variation of threshold voltage, ΔVT, from device to device. The effect of such variation in an OELD display, exacerbated by the non perfect diode behaviour, is the non-uniform pixel brightness over the display panel, which seriously affects the image quality. Therefore, a built-in compensation circuit is required.
A simple threshold voltage variation compensation, current driven, circuit has been proposed. The current driven circuit, also known as the current programmed threshold voltage compensation circuit is illustrated in FIG. 2A. In this circuit, T1 is for addressing the pixel. T2 operates as an analog current control to provide the driving current. T3 connects between the drain and gate of T2 and toggles T2 to be either a diode or in saturation. T4 acts a switch. Either T1 or T4 can be ON at any one time. Initially, T1 and T3 are OFF, and T4 is ON. When T4 is OFF, T1 and T3 are ON, and a current of known value is allowed to flow into the OLED, through T2. This is the programming stage because the threshold voltage of T2 is measured with T2 operating as a diode (with T3 turned ON) while the programming current is allowed to flow through T1, through T2 and into the OELD. T3 shorts the drain and gate of T2 and turns T2 in to a diode. The detected threshold voltage of T2 is stored by the capacitor C1 connected between the gate and source terminals of T2 when T3 and T1 are switched OFF. Then T4 is turned ON, the current is now provided by VDD. If the slope of the output characteristics were flat, the reproduced current would be the same as the programmed current for any threshold voltage of T2 detected. By turning ON T4, the drain-source voltage of T2 is pulled up, so a flat output characteristic will keep the reproduced current the same as the programmed current. Note that ΔVT2 shown in FIG. 2A is imaginary, not real.
A constant currant is provided, in theory, during the active programming stage, which is t3 to t4 in the timing diagram shown in FIG. 2A. The reproduction stage starts at t6 and ends at t1 of the next cycle.
In practice, there is always a slope in the output characteristics, so the reproduced current is not the same as the programmed current. This issue limits the device channel length of the polysilicon TFTs because of the increase of the short channel effect in polysilicon TFTs when the device channel length gets smaller. Simulations show that the variation between the reproduced current and programmed current is unacceptable for L=4 μm and below. This limitation on the design of transistor T2 is a very serious practical problem, especially when small data currents are used. It is therefore important to find a technique that will provide good compensation in short channel devices.
The driving waveforms used are shown in timing chart fashion in FIG. 2B. The threshold voltage VT shown at the bottom of FIG. 2B is that for transistor T2. As can be seen from FIG. 2B, this threshold voltage has a range of −1V to +1V. Such a range is much larger than the variation ΔVT across a practical OELD mate.
Typical variation between the reproduced current and programmed current supplied to the OELD is illustrated in FIG. 2C. FIG. 2C illustrates three cycles of OELD current supply: one from 0 to 30 μs, one from 30 μs to 60 μs, and one from 60 μs to 90 μs. The first half of each of these cycles is the programming stage and the second half of the cycle is the reproduction stage. It is to be noted that the current output levels in the reproduction stage compared with those in the corresponding program stage are remarkably different firm each other.